1. Field of the Invention
The present invention relates to a liquid crystal display device and its manufacturing method, and particularly relates to an active matrix type liquid crystal display device having a color filter on its element substrate and its manufacturing method.
2. Description of the Related Art
The active matrix liquid crystal display device comprises an element substrate with a switching element such as a transistor, etc., an opposite substrate arranged to be opposite to the element substrate, and a liquid crystal filled between the element substrate and opposite substrate.
The opposite substrate has a color filter (CF) and a black matrix. The color filter consists of color layers of red, green or blue arranged regularly. The black matrix is formed of a film with a light shielding property (opaque). The black matrix shields disclination of the liquid crystal to prevent a reduction in contrast.
The liquid crystal is filled in a space between the element substrate and opposite substrate. Alignment layers are formed on the opposite surfaces of the element substrate and opposite substrate respectively. The alignment layers are formed in such a manner that a resin film such as polyimide is formed on substrates and the resultant surfaces are subjected to alignment treatment such as rubbing. The alignment layers provide a predetermined alignment (orientation) to liquid crystal molecules in the liquid crystal.
Here, the disclination refers to an alignment defect in which discontinuity occurs in the alignment direction of liquid crystal molecules due to a level difference in the alignment surface of liquid crystal, distribution of an electric filed and an abrupt change in a driving voltage. Such liquid crystal alignment defect appears as a line or dot detect, resulting in deterioration of display quality. The disclination occurs in a display area overlapping with irregular portions on the surfaces of substrates, non-uniform portions of the liquid crystal alignment caused by variations in the degree of rubbing, and the like.
In the above structure in which the color filter and black matrix are provided on the opposite substrate, it is necessary to preform the black matrix with a width larger than that of an ideal masking area on design. The reason lies in ensuring a margin for error in the alignment of element substrate and opposite substrate. However, if the width of the black matrix is too wide, there occurs difficulty in setting an aperture ratio of liquid crystal display element largely, thereby decreasing brightness. Here, the aperture ratio means the percentage of a pixel area contributing to optical modulation to the entire surface area of the display area of a liquid crystal display panel.
In order to improve the aperture ratio of liquid crystal display element, there is disclosed the structure for forming a color filter and black matrix on an element substrate. This is referred to as CF (Color Filter)-On TFT structure, and the structure is described, for example, in Japanese Patent No. 2758410 and Unexamined Japanese Patent Application KOKAI Publication No. H3-237432.
In the CF-On TFT structure, the color filter and black matrix are formed on the element substrate. Accordingly, it is unnecessary to ensure a margin for alignment of the element substrate and opposite substrate. This makes it possible to simplify the manufacturing process and to achieve a high aperture ratio.
At the occasion of pushing forward high definition, the use of CF-On TFT structure makes it difficult to realize the high aperture ratio for the following reason.
More specifically, in the CF-On TFT structure, the size of contact hole for connecting a pixel electrode and a source electrode is relatively large, and an exclusive area for a contact hole accounts for a relatively high percentage in each pixel area. This is because the taper of the side surface of contact hole must be gently formed to prevent occurrence of level separation of the pixel electrode formed on the inner surface of contact hole. In order to make the taper gentle, a plurality of layers (passivation layer, color layer, overcoat layer, etc.), which separate the pixel electrode and source electrode, is patterned to form apertures respectively. In this case of forming a contact hole for each layer, it is necessary to ensure a margin for alignment for each patterning process. Resultantly, the size of contact hole is relatively large. The area for contact hole is generally shielded from light. Accordingly, the percentage of the area for the contact hole in the pixel area increases with the progress of high definition, resulting that the aperture ratio relatively decreases.
Moreover, the percentage of the area for the black mask for masking (shielding) the disclination occurring region relatively increases with the progress of high definition, though this is not limited to the CF-On TFT structure. Accordingly, the aperture ratio relatively decreases.
Thus, in the active matrix liquid crystal display device, even though the CF-On TFT structure is used, the percentage of masking area relatively increases, resulting in decreasing in the aperture ratio, if the high definition progresses and a pixel pitch becomes narrow. On the other hand, if the percentage of masking area is small in order to achieve high aperture ratio, disclination becomes conspicuous.